How many diagonal in the icosagon?

An icosagon, which is a polygon with 20 sides, has a total of 170 diagonals. The formula to calculate the number of diagonals in a polygon is ( \frac{n(n-3)}{2} ), where ( n ) is the number of sides. For an icosagon, this calculation is ( \frac{20(20-3)}{2} = \frac{20 \times 17}{2} = 170 ).